1. Field of the Invention
The present invention relates to a nonvolatile semiconductor memory, for example, a flash memory which performs data writing using FN tunneling.
2. Description of the Related Art
There are common problems in a flash memory performing data writing using an FN tunneling, that is, a leak current caused by an inter-band tunneling at an unselected cell at the time of data writing and charge retention characteristics of a memory cell.
The leak current generated at the unselected cell lowers step-up capability of a step-up circuit generating high voltage; therefore there is a danger that the data writing itself cannot be performed. Further, as miniaturization of a memory cell is advanced, a leak rate of a charge within a floating gate electrode becomes high, and retention period of data (charge) becomes short.
As one method for solving such problems, a technique is known which rounds an edge of the floating gate electrode of the memory cell and an edge of a select gate electrode of a select gate transistor due to thermal oxidation (for example, referring to Jpn. Pat. Appln. KOKAI Publication Nos. 6-310731, 7-249697, 10-65028, and 10-284626).
A degree of roundness of the edge is specified with a size of a bird's beak because a shape of a thermal oxide film formed in the edge is similar to the bird's beak. When enlarging the bird's beak, it is possible to realize the leak current prevention caused by the inter-band tunneling and improvement of charge retention characteristics.
However, when the bird's beak of the floating gate electrode of the memory cell becomes large, an average thickness of a tunnel oxide film (gate insulating film) becomes large, so that the writing characteristics, that is, efficiency for injecting charge into the floating gate electrode from a channel by an FN tunneling degrades.
Thus, conventionally, in the course of advancing miniaturization of the memory cell of the flash memory, it is not possible to realize simultaneously the prevention of the leak current caused by the inter-band tunneling and the improvement of the charge retention characteristics without deteriorating the writing characteristics.